Here, ‘transmission’ denotes a multi-gear transmission in which a predefined number of gears, i.e. fixed transmission ratios between the transmission input and the transmission output, can be engaged automatically by means of shifting elements. The shifting elements are for example clutches and/or brakes.
At present, as drive systems for electric vehicles single-gear transmissions are mainly used, i.e. simple step-down stages with an output rotational speed lower compared with the input rotational speed. With such a structure, however, the electric drive must cover the complete rotational speed range and this reduces its overall efficiency. Furthermore, the rotational speed range that can be realized is rather narrow.
It has therefore already been proposed to use two-gear transmissions as drive systems for electric vehicles. However, until now these have been designed as conventional manual-shift transmissions with sliding sleeves. If such a transmission is used the electric machine can be made smaller, and this results in a clear cost advantage. However, precisely because the transmission is made with sliding sleeves, there is a traction force interruption during the shift. For automated drive-trains, however, such a traction force interruption results in a considerable impairment of comfort, which is undesired by the customer or the driver.
DE 10 2010 024 147 A1 discloses a two-gear transmission for transmitting drive torque in a drive-train of a vehicle, preferably for use in a drive-train for electric vehicles, in a drive-train for hybrid vehicles or in an auxiliary drive-train for hybrid vehicles, by virtue of such a transmission, gearshifts can be carried out without traction force interruption or with less traction force interruption, by virtue of two automated and controllable clutches.
DE 10 2010 061 054 A1 discloses a drive-train for a hybrid vehicle, with which three or more fixed transmission ratio modes can be produced, this drive-train comprises a first and a second motor-generator, two individual planetary gearsets and four torque-transmitting elements.